A total of 341 stool specimens known to be positive for group A rotaviruses by RNA electrophoresis, obtained from various locations in Malaysia from hospitalized children below the age of five years who had gastroenteritis, was brought to the NIH for determination of the serotypes circulating in Malaysia and for further characterization by molecular biologic techniques. Most of the specimens (87%) were collected in the 1987/88 season; the remainder (13%) were collected in 1977, 78, 83, 84, and 86. Serotype analysis was performed on 306 specimens; 35 specimens were antigen-negative when tested by a pre-post confirmatory ELISA and were, thus, not analyzed. One hundred eighty-two of the 306 specimens (60%) could be serotyped: 15% were serotype 1, 4% serotype 2, 4% serotype 3, 71% serotype 4, and 6% exhibited multiple serotype-specificities. Most of the typable specimens (93%) were from the 1987/88 season. The remaining 7% came from 1978, 1984, and 1986 (none were typed from 1977 and 1983). Serotype 4, followed by serotype 1 rotavirus, predominated in central and north-western Peninsular Malaysia in the 1987/88 season, while it appeared from a limited number of strains that a more even serotype distribution was observed in East Malaysia. Eighty-two percent of stool specimens that were serotyped could be adapted to grow in primary African Green monkey kidney (AGMK) cells after one or two passages, whereas only 44% of the specimens that did not serotype were adapted successfully. It was noteworthy that 32% of rotavirus ELISA-negative stools were adapted to growth in AGMK cells and found to be rotavirus positive by ELISA. Of 190 specimens that grew after second passage in AGMK cells, 78% could be adapted to growth in MA104 cees after one passage; 10% of these were specimens that did not serotype. The latter, plus two possible serotype 1 monotypes and specimens that reacted with more than one serotype-specific monoclonal antibody are potential candidates for molecular characterization of their VP7 genes. Some of these are being amplified in MA104 roller cultures. In addition, all rotavirus ELISA-positive MA104 passages will be subjected to VP4 typing to determine the distribution of VP4 serotypes in Malaysia and to detect potential candidates for virus amplification and VP4 gene sequencing.